The purpose of this study is to localize and characterize the prolactin receptor site(s) in/on rat luteinizing granulosa cells employing morphological and biochemical techniques. To this end, granulosa cells will be obtained for 5iu PMSG-treated immature rats, and cultured under optimal conditions for the induction of prolactin receptors. In initial studies, cells of increasing densities will be incubated with 125I-b,oPRL under varying states of receptor differentiation to determine optimal conditions of time, temperature and pH for high affinity and specific binding of labeled hormone to PRL receptors. Scatchard analyses will be undertaken. Once these conditions have been determined, cells and sections (frozen and plastic) of cells will be incubated with 125I- or ferritin-labeled PRL to localize the distribution pattern of PRL receptors by autoradiography and electron microscopy. In other studies, cells and sections will be subjected to various immunocytochemical reagents in an attempt at localizing the PRL receptor(s). Every effort will be made to control for non-specific binding and the degree of binding of PRL will be correlated with the capacity of hormone (labeled or unlabeled) to promote progesterone secretion by living cells.